Electron multipliers are components that provide amplification. Their gain is generally very high, and may be as much as 10.sup.4 or 10.sup.5. They are used in various applications where very weak signals are processed. For example, such applications include: measuring light energy in the visible or the invisible portions of the spectrum (photomultipliers), image intensifiers, measuring nuclear radiation (ionization chambers), mass spectrometry, and in particular detecting leaks by means of helium.
In these applications, the signals to be processed may have a dynamic range of about 10.sup.8, in some cases. It is therefore necessary to have a measurement system which includes the electron multiplier and an amplifier for compressing the signal. The signal-compressing amplifier cannot, on its own, reduce the output dynamic range sufficiently. It is therefore necessary to act on the gain of the electron multiplier in order to a achieve additional compression. This is done by acting on its power supply voltage. In known circuits, the gain of the measurement system, in particular due to the action taken on the said power supply voltage, is controlled discontinuously either manually by using a switch or else, in more sophisticated systems, by automatic electronic switching, or else multiple measurement systems are used. This discontinuity in transfer function gives rise to undesirable effects such as long response times or instability whenever the signal is near the boundary between two amplification ranges, or else measurement curves join up incorrectly.
The object of the present invention is therefore to provide an improved circuit for processing a signal received by an electron multiplier, which circuit mitigates the above drawbacks.